Specify a first point on the mirror axis with the mouse or enter a coordinate in the command line.

Specify a second point on the mirror axis.

The mirror dialog is displayed. To mirror the entities without keeping the original entities, choose "Delete Original", to copy them choose "Keep Original". The new entities are placed on the same layer as the originals and have the same attributes. To use the current layer and current attributes instead, tick "Use current layer and attributes".